Run multiple worker loops

[matthew-white]

Closes #533. Instead of running a single worker loop, Backend will now run 4. I didn't change the timing of how a worker waits between work.

What has been done to verify that this works as intended?

Existing tests continue to pass, but tests usually run workers manually (via exhaust()). When they do run a loop, it's at most one loop. I think it's hard to really test multiple loops and that the best thing to do is just to get the PR merged before regression testing begins. If we run into performance or other issues during regression testing, we can address them then.

Why is this the best possible solution? Were any other approaches considered?

I probably could have called the existing worker() function multiple times, but I noticed that each invocation of worker() would create functions to check, run, and report. Instead, I created a new function named workerQueue() that will create all worker-related functions at once. That reduces the number of duplicate functions and I think is a little easier to understand.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

We already have concurrent workers in place because of our use of pm2, so I'm hoping we won't run into any issues.

Before submitting this PR, please make sure you have:

• run make test-full and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced